UCP1 and obesity due to melanocortin 4 receptor deficiency: Loss of FGF21 prevented the browning, i.e., induction of thermogenic adipocytes in inguinal and visceral white adipose tissue (WAT) and also had a minor effect on circulating triglycerides, but it did not prevent obesity resistance and protection from developing a fatty liver, nor the improved insulin sensitivity observed in UCP1-tg mice [119].